Conventionally, there has been a technique to include curvature information of curves into a map Database (DB) and thereby present caution information to drivers before the drivers travel a curve having a large curvature, or a technique to present caution information to drivers on the basis of a relation between a travel speed and a curvature of a curve when caution is required to travel the curve.
Further, disclosed has been a technique to determine whether or not a road surface is slippery, on the basis of vehicle information, and present caution information to drivers when the road surface is slippery (see, for example, Patent Document 1).